For bone fractures such as peri-articular fractures, bone plating systems that provide fixed angular relationship between the bone plate and the bone screw are known to be beneficial. Many of these devices require suitable bone quantity and quality, and a fracture pattern that is compatible with the device to be effective. Where these requirements are not satisfied, e.g. bone fractures involving severely comminuted bone or missing bone segments, conventional bone plate and screw systems must be used. Although these conventional systems are particularly well-suited to promoting healing of the fracture by compressing the fracture ends together and drawing the bone into close apposition with other fragments and the bone plate, the angular relationships between the plate and screws are not fixed and can change postoperatively. This can lead to malalignment and poor clinical results.
The primary mechanism for the change in angular relationship between the bone plate and the bone screws is the movement between the bone plate and the compressed bone caused by the dynamic loading from physiological conditions. The shear forces generated by the dynamic loading often cause the bone plate and screws to come loose and release the stored energy in the compressed bone.
Securing or locking the bone screws to the bone plate provides a fixed angular relationship between the plate and screw and reduces the incidence of loosening. One method of securing the bone screws to the bone plate involves the use of locking screws. A locking screw has threading on an outer surface of its head that mates with corresponding threads on the surface of a plate hole to lock the screw to the plate. Bone plates having threaded holes for accommodating locking screws are known. As the relationship between the locking screws and the bone plate is fixed, locking screws provide a high resistance to shear or torsional forces. However, locking screws have a limited capability to compress bone fragments.
In summary, conventional non-locking bone screws, i.e. screws that do not lock in a fixed angular relationship to a bone plate, effectively compress bone fragments, but generally possess a low resistance to shear forces that can lead to loosening of the screw. Locking screws, on the other hand, have a high resistance to shear forces ensuring stability at the bone screw-to-plate hole interface, but possess a limited ability to compress the bone fragments. Thus, a bone plating system that combines non-locking screws with locking screws in long bone applications has been introduced in the industry. One such example is a bone plating system disclosed in U.S. Pat. No. 6,623,486 to Weaver et al.
Thus, it can be appreciated that there exists a continuing need for a new and improved bone plating system that would enhance the effectiveness and useability of these bone plating systems.